In an imaging device such as a digital camera for picking up an image by converting an image signal into an electric signal, a photographing light flux is received by an image pickup element such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Then, a photoelectric conversion signal output from the image pickup element is converted into image data, and the image data is recorded on a recording medium such as a memory card. In such an imaging device, an optical low-pass filter and an infrared cut filter are disposed in front (on a subject side) of the image pickup element.
In the imaging device of this type, when dust adheres to a surface of a cover glass of the image pickup element or to surfaces of those filters, the dust may appear as a black spot in the picked-up image. In particular, in a single-lens reflex digital camera which includes an interchangeable lens, dust may enter the digital camera main body through the opening of the lens mount when the lens is being changed, and may adhere to the surfaces of the cover glass of the image pickup element or of the filters.
In view of the above, there has been proposed a digital camera which includes a dust removing device for removing dust adhering to the surface by using a vibration of a piezoelectric element (see, for example, PTL 1).
In the dust removing device included in the digital camera disclosed in PTL 1, an alternating voltage having a predetermined frequency is applied to a piezoelectric element fixedly bonded to a vibration member (dust filter in PTL 1) so as to drive the piezoelectric element, to thereby generate elastic vibration (hereinafter, defined as flexural vibration) for causing a displacement of the vibration member in an optical axis direction, that is, in a thickness direction of the vibration member. The dust removing device of PTL 1 removes dust adhering to the surface of the vibration member through application of the flexural vibration thus generated.
In the digital camera of PTL 1 having the above-mentioned configuration, a front surface side of the vibration member is held by a pushing member one end of which is fixed to a camera main body (imaging device) by a screw, and a rear surface side of the vibration member is held by a seal provided on the camera main body (imaging device). On the rear surface side of the vibration member, a sealed state is ensured by the vibration member, the seal, and the camera main body (a sealed space for preventing a foreign matter such as dust from entering therein is formed), and hence, dust is not projected on a picked up image only if dust on the front surface side of the vibration member can be removed.
In this case, the vibration member is pushed against the seal with a pressure from the pushing member. The pushing member made of a plate material of phosphor bronze for a spring or stainless steel for a spring has high bending stiffness, and the seal made of a rubber material has low bending stiffness, and hence, the seal is deformed with a pressure or an external force (inertia force, etc.).
Further, a receiving member bonded to the pushing member is interposed between the pushing member and the vibration member, and the receiving member receives the vibration member through the intermediation of a positioning member to position the vibration member in a plane perpendicular to an optical axis. The receiving member and the positioning member are made of materials having vibration damping such as rubber and a resin.
Further, a receiving portion is set on the rear surface side of the vibration member. The stiffness of the receiving portion having vibration damping, made of rubber, a soft resin, or the like, is higher than that of the seal, and hence, the seal is prevented from being deformed with respect to a certain pressure or more of the vibration member.
Incidentally, the receiving portion is arranged so as to support a node of the vibration member in which flexural vibration hardly occurs so as not to inhibit the vibration of the vibration member.
A piezoelectric element fixedly bonded to the vibration member is electrically connected to a flexible printed board for applying the alternating voltage having the predetermined frequency to the piezoelectric element, and this connection is performed generally through adhesion with an anisotropic conductive film (ACF) or a resin.
As described above, the dust removing device requires a fixation member for setting the dust removing device on a base such as an imaging device or a camera main body. The fixation member requires damping ability, as its function, so as not to transmit excess vibration to the base. On the other hand, as shown in a series of configurations as in PTL 1, the fixation member has an aspect of remarkably inhibiting the vibration of the vibration member to degrade the effect of removing dust. Therefore, a soft high molecular compound material such as rubber or a resin is used for the fixation member.
Further, in the dust removing device, a soft material such as a resin is used also for fixed bonding between the piezoelectric element and the vibration member and adhesion between the piezoelectric element and the flexible printed board.
The piezoelectric element of PTL 1 is in the form of a plate having a ring shape or a rectangular shape, and includes a piezoelectric material and a pair of electrodes opposed to each other. The pair of electrodes includes a first electrode and a second electrode which are disposed on plate surfaces of the piezoelectric material. The first electrode is also called lower electrode, and the second electrode is also called upper electrode. Here, an expanding and contracting distortion is generated in the piezoelectric material by an electric field applied between the electrodes, which generates an elastic vibration (hereinafter, defined as length vibration) for causing a displacement of the piezoelectric element of PTL 1 in a direction perpendicular to the optical axis of the vibration member, that is, in a direction (hereinafter, defined as length direction) perpendicular to the thickness direction of the piezoelectric element. Due to the length vibration of the piezoelectric element, a stress is generated between the piezoelectric element and the vibration member fixedly bonded to the piezoelectric element, to thereby generate a flexural vibration in the vibration member.
In the vibration member, a voltage to be applied to the piezoelectric element is controlled in frequency or phase so that the flexural vibration of the vibration member may generate a standing wave of multiple orders, which is called vibration modes, having multiple node portions and antinode portions, or a traveling wave which has node portions and antinode portions and moves in a length direction of the vibration member with respect to time. For example, in the dust removing device included in the digital camera disclosed in PTL 1, a pair of the piezoelectric elements are applied with voltages which are controlled in phase so that multiple vibration modes may be generated, and the multiple vibration modes are selectively used effectively, to thereby effectively remove dust adhering to a surface of the vibration member.
Here, the dust removing device of PTL 1 is driven at a frequency in proximity to the resonance frequency of the vibration member, and hence a larger flexural vibration may be generated in the vibration member even when a smaller voltage is applied to the piezoelectric element.
Further, the dust removing device of PTL 1 is configured so as to remove dust without any problem with respect to an individual variation of the dust removing device and a fluctuation in frequency with respect to temperature, by sweeping a frequency band in the vicinity of a resonance frequency. According to PTL 1, the frequency for oscillating the piezoelectric element is determined by the shape, size and material, and the support state of the piezoelectric element and a vibrator formed of a vibration member, and generally, the temperature is one factor for influencing the coefficient of elasticity of the vibrator and changing the resonance frequency thereof. Thus, the temperature is one factor for changing the resonance frequency of the vibrator and the dust removing device.
The magnitude of length vibration of the piezoelectric element is closely related to the magnitude of piezoelectric displacement caused by a traverse piezoelectric effect of a piezoelectric ceramics, and hence, a piezoelectric element excellent in piezoelectric characteristics is chosen.
Meanwhile, the piezoelectric element which is currently used in various devices uses a piezoelectric material containing a large amount of lead, such as lead zirconate titanate (PZT: PbZr1-xTixO3) containing lead, in many cases. For example, the piezoelectric element of PTL 1 uses lead zirconate titanate. However, it has been pointed out that such a piezoelectric material made of lead zirconate titanate containing a large amount of lead can be detrimental to ecosystems because the lead component in the piezoelectric material seeps into soil when the piezoelectric material is once discarded to be exposed to acid rain, for example. In view of this, in recent years, with consideration given to the environment and to comply with laws restricting the use of lead in various products, a piezoelectric material (lead-free piezoelectric material) containing no lead or a minimum amount of lead and product development therefor are under study and consideration. However, a prominent lead-free piezoelectric material having various properties equivalent to those of lead zirconate titanate has not been realized, and there are still only a few examples of commercialized devices using a lead-free piezoelectric material which is equivalent in quality to lead zirconate titanate.